PEXT is getting a major refactoring

[PhilippvK]

It seems like most of the proposed RVP extensions are getting renamed to be more consistent overall.

Examples:

• ADD8 -> ADD.B
• ADD16 -> ADD.H

See http://www.jhauser.us/RISCV/ext-P/RVP-baseInstrs-A-002.pdf for more details.

In addition there will also be be new instructions added to the spec and some of the proposed ones might be dropped. Since none of this is finalized I would not put effort into changing muRISCV-NNs PEXT implementation because it might change again later. We should really wait for it’s ratification and until a proper intrinsics API + usable toolchain (GCC/LLVM) is available.

[PhilippvK]

Since Spike recently dropped support of the RVP draft, we can will also pause running unit/integration tests for RVP until we have a reference simulator again